This invention relates to apparatus for receiving and analyzing particulate matter contained in air or other gases. More particularly the invention relates to virtual impactors for dividing a gas flow into one flow component containing particles larger than a predetermined cutpoint size and another flow component carrying particles of less than the cutpoint size.
Airborne particulate matter variously includes natural aerosals such as soil dust, pollens and sea spray and a large number of other pollutants, such as hydrocarbons, sulphates and nitrates originating from human activities. Particles having aerodynamic sizes below about 15 microns are inhalable by humans and may produce adverse physiological effects. Monitoring of the particulate contaminants of atmospheric air, particularly in urban and industrialized areas, is therefore desirable in order to determine the nature, origin and severity of these effects and to assist in the development of countermeasures and controls.
The inhalable particles present in urban aerosals exhibit a bimodal size distribution having a minimum particle concentration at about 2.5 microns. The fine particle component, having sizes smaller than that value, tends to have a maximum concentration at a size of about 0.3 micron and is primarily derived from combustion products through condensation and coagulation. The coarse particles, having sizes larger than 2.5 microns are primarily of natural origin or are mechanically produced and are typically found in their greatest concentration at sizes around 10 microns.
This bimodal size distribution coincides at least approximately with distinct differences in certain physiological effects of the airborne particles. The fine particles, for example, are much less efficiently removed in the human nasal-pharyngeal regions and therefore penetrate more easily into the tracheo-bronchial and pulmonary regions of the human body.
Thus there are significant differences between the fine and coarse airborne particles with respect to origin, chemical properties, health effects and environmental impact. Accordingly, monitoring and analysis of atmospheric contamination on a continuing basis can be more effective if the fine and coarse particles are separated for separate collection and analysis.
Devices which are commonly used for separating an air sample into first and second flow components, respectively carrying the coarse and fine particles for separate collection and analysis, rely on inertial separation and are referred to as impactors. In one known form, air is drawn by a pump into a particle collection chamber through an inlet flow passage that is directed toward an impaction plate. The particle flow divides at a separation region between the inlet passage and plate with the fine particles being drawn away from the axis of the flow and into the collection chamber while the heavier coarse particles diverge from the axis less strongly and are collected by impaction on the plate. A significant problem with such impactors is that the coarse particles tend to bounce off the impaction plate and become reentrained in the air flow into the fine particle collection region. Deposition of the coarse particles on the impaction plate also tends to be highly nonuniform. This is not compatable with preferred methods of particle analysis such as mass measurement by Beta attenuation and elemental composition determination by x-ray florescence analysis.
These problems have been reduced to some extent by another known type of flow separation device which is commonly referred to as a virtual impactor. In a virtual impactor, the impaction plate is replaced by an annular opening defined by one end of a coarse particle collection probe which receives the coarse particle flow component.
A virtual impactor has several distinct advantages relative to impaction plate devices. Rather than detracting from precision, particle bounce from wall surfaces is a favorable phenomenon in that it reduces losses. As both classes of particles are collected on filters situated a distance away from the flow separation region, sample handling, including automatic sample processing if desired, is facilitated. Particle deposition on collection filters is more uniform and this facilitates x-ray florescence analysis of the collected particles.
As heretofore constructed, virtual impactors do not fully resolve the above discussed problems. Particles with sizes near the cutpoint size tend to impact against structural surfaces around the flow separation region and are effectively lost by adherence to such surfaces or through combining with other impacted particles. Thus prior virtual impactors have tended to exhibit an undesirably high particle loss peak near the cutpoint. Other particles, particularly of the largest sizes, tend to be lost in the inlet flow. The inlet flow progressively increases in velocity as it is decreased in cross-sectional area and the larger, heavier particles resist the necessary changes of trajectory and tend to impact against the wall of the inlet means. Other wall losses occur because of gravitational settling against wall surfaces of the impactor.
To achieve reasonable separation efficiency, prior virtual impactors have required at least two stages of size separation. This aggravates the particle loss problem and such devices typically have a loss peak of about 25 percent at the cutpoint size and of about 60 percent at the large 20 microns size for liquid particles. The complexity and the cost of the instrument are also undesirably large. Further, uniformity of deposition of particles on downstream collection filters has been less than would be desirable for certain method of analysis such as x-ray florescence analysis.